Lecture Notes Chapter 45 - Chapter 45: Hormones and the...

Chapter 45: Hormones and the Endocrine System Overview: The Body’s Long-Distance Regulators An animal hormone is a chemical signal that is secreted into the circulatory system and communicates regulatory messages within the body Hormones may reach all parts of the body • But only certain types of cells, target cells, are equipped to respond Examples: Insect metamorphosis is regulated by hormones – ecdyson is the universal molting hormone in arthropods (steroid) The endocrine system and the nervous system act individually and together in regulating an animal’s physiology The nervous system • Conveys high-speed electrical signals along specialized cells called neurons The endocrine system (made up of endocrine glands) • Secretes hormones that coordinate slower but longer-acting responses to stimuli Overlap Between Endocrine and Nervous Regulation The endocrine and nervous systems often function together in maintaining homeostasis, development, and reproduction • Specialized nerve cells known as neurosecretory cells release neurohormones into the blood • Both endocrine hormones and neurohormones function as long-distance regulators of many physiological processes Control Pathways and Feedback Loops There are three types of hormonal control pathways: • A common feature of control pathways is a feedback loop connecting the response to the initial stimulus • Negative feedback regulates many hormonal pathways involved in homeostasis • Hormones and other chemical signals bind to target cell receptors, initiating pathways that culminate in specific cell responses • Hormones convey information via the bloodstream to target cells throughout the body Three major classes of molecules function as hormones in vertebrates: • Proteins and peptides (e.g. insulin, growth hormone) • Amines derived from amino acids (e.g. adrenalin) • Steroids (testosterone, estrogen) 1

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Signaling by any of these molecules involves three key events: 1. Reception – The possession of the appropriate receptor determines whether a cell can respond to the stimulus 2. Signal transduction – Translation into the cellular “language” • Often involves an amplification step 3. Response – At the cellular level the response to a hormone usually involves changes in the expression of certain genes There is a principle difference in transduction of water-soluble and lipid-soluble hormones, since the latter can readily cross a lipid bilayer, whereas peptides cannot. Cell-Surface Receptors for Water-Soluble Hormones The receptors for most water-soluble hormones are embedded in the plasma membrane, projecting outward from the cell surface Binding of a hormone to its receptor initiates a signal transduction pathway leading to specific responses in the cytoplasm or a change in gene expression The same hormone may have different effects on target cells that have • Different receptors for the hormone • Different signal transduction pathways

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